The present invention generally relates to semiconductor devices, and, more particularly, to a semiconductor device that has an oxide film formed by a silicon substrate and an epitaxial film grown on the silicon substrate.
Conventionally, the formation of an oxide film on a silicon substrate has been commonly performed. For most cases, the oxide film is an amorphous film, and is mainly employed as an insulating film or a dielectric film.
In a semiconductor device that utilizes the properties of an oxide film, such as a ferroelectric memory, a crystallized oxide film is employed to realize desired functions. Some oxide crystals exhibit many properties including not only insulation and dielectric properties but also ferroelectricity, piezoelectricity, pyroelectricity, and superconductivity. By forming oxide crystals having these properties as a thin film on a single-crystal silicon substrate, a device having various functions such as a memory, a sensor, and a filter, can be obtained. These functions derive from the properties of the oxide crystals. In an amorphous state, however, the oxide films cannot exhibit the properties or can exhibit only a part of the properties.
A ferroelectric film used in a ferroelectric memory obtains the above properties through crystallization in the existence of oxygen at a temperature of several hundred degrees centigrade. However, a conventional ferroelectric film is a polycrystalline film, in which the orientations of crystals in a direction perpendicular to the substrate, for instance, are aligned, but the orientations of crystals in other directions are generally at random, resulting in defects with the grain boundaries, for instance. For this reason, a semiconductor device including a conventional crystalline oxide film only has a limited ability to exhibit the properties of the oxide crystal.
Meanwhile, it has been very difficult to form an oxide film having an epitaxial orientation in which the crystal orientations are aligned not only in a direction perpendicular to the substrate surface but also in a direction parallel to the substrate surface.
To develop an epitaxial oxide thin film on a single-crystal silicon substrate, it is necessary to utilize the orientations on the surface of the single-crystal silicon substrate. However, a single-crystal silicon substrate has the same chemical properties as metals. If exposed to an oxygen atmosphere at a high temperature, the surface of a single-crystal silicon substrate is quickly oxidized to form a silicon oxide (SiOx) film. Since a silicon oxide film is amorphous and does not have a crystal orientation, an epitaxial oxide thin film cannot grow on a silicon oxide film. It is also essential for epitaxial growth that the reaction and diffusion between a growing thin film and a single-crystal silicon substrate should be minimized. For this reason, not all oxides can be formed through epitaxial growth on a single-crystal silicon substrate. Materials that are known to date as suitable for epitaxial growth on a single-crystal silicon substrate only include rare earth element oxides such as yttrium fully stabilized zirconia (YSZ: see J.Appl.Phys. vol. 67, (1989) pp. 2447), magnesia spinel (MgAl2O4: ISSCC Digest of Tech. Papers (1981) pp. 210), and cerium oxide (CeO2: Appl.Phys.Lett, vol. 56, (1990) pp. 1332), and strontium titanate (StTiO3: Jpn.J.Appl.Phys, 30 (1990) L1415).
The index of crystal quality of an epitaxial oxide thin film formed on a silicon substrate is a half value width that is obtained through X-ray diffraction (Full Width at Half Maximum, FWHM). A half value width is a value determined from a rocking curve obtained by scanning fixed 2 xcex8 axes of an X-ray peak. More specifically, the half value width (FWHM) is determined by the peak width at a half of the strength of the peak top of the rocking curve. This indicates the degree of crystal tilt in the thin film. A smaller value indicates properties similar to those of a single-crystal material, which exhibits better crystallization and orientation. With aligned orientations of crystals in a thin film, the electric properties (such as leak properties with improved hysteresis properties) are improved. It is therefore essential that a thin film having as small half value width (FWHM) as possible should be produced for suitable application to a device.
Materials having perovskite structures, including barium titanate, are ferroelectric materials that are desirable in terms of piezoelectricity, dielectricity, pyroelectricity, semiconductivity, and electric conductivity. However, it has been conventionally difficult to develop a material having a perovskite structure through epitaxial growth directly on a single-crystal silicon substrate. This is because an amorphous-phase SiOx film or a reaction phase such as silicide is formed on a single-crystal silicon substrate.
The only epitaxial perovskite thin film conventionally employed and formed on a single-crystal silicon substrate is strontium titanium (SrTiO3). A metal strontium film as an intermediate layer is interposed between a strontium titanium thin film and a single-crystal silicon substrate. Since titanium and silicon are reactive to each other, a strontium titanate film is formed to prevent reaction between titanium and silicon. More specifically, after a metal strontium film is formed on the surface of a silicon substrate, strontium and titanium are supplied in the existence of oxygen so as to produce a strontium titanate film. If the metal strontium film as an intermediate layer is thin enough, the titanium diffuses into the metal strontium film during the formation of a SrTiO3 film. As a result, a SrTiO3 film that appeases to have developed through epitaxial growth directly on the single-crystal silicon substrate can be obtained.
To develop a strontium titanate film through epitaxial growth in the above manner, a process control is necessary at the atomic layer level, and, therefore, a molecular beam epitaxy (MBE) technique is employed. Alternatively, Japanese Laid-Open Patent Application No. 10-107216 discloses a method of forming a strontium titanate (SrTiO3) film. More specifically, high-vacuum laser ablation is performed on a SrO target in a high vacuum of 10xe2x88x928 Torr, so as to form a strontium oxide (SrO) film as an intermediate layer. A strontium titanate (SrTiO3) film is then formed on the SrO film. If the SrO intermediate layer is thin enough, the titanium diffuses into the SrO intermediate layer during the formation of the SrTiO3 film. As a result, it appears that the SrTiO3 film has developed through epitaxial growth directly on the single-crystal silicon substrate.
Alternatively, another method has been suggested in which an intermediate layer is formed to prevent the reaction between a single-crystal silicon substrate and a perovskite oxide, and the formation of a SiOx phase. Intermediate layers that are known to date include yttria partially stabilized zirconia (YSZ: J.Appl.Phys. 67 (1989) pp. 2447) and magnesia spinel (MgAl2O4: ISSCC Digest of Tech. Papers (1981) pp. 210). With any of these intermediate layer, a 2-layered structure in which the intermediate layer and a perovskite film are laminated in this order on a single-crystal silicon substrate is obtained.
A yttria partially stabilized zirconia (YSZ) thin film formed through epitaxial growth on a single-crystal silicon substrate is obtained by a pulse laser deposition technique using YSZ ceramics as a target. Where a perovskite film is formed on such a YSZ film on a single-crystal silicon substrate, an epitaxial phenomenon in which the (011)-plane of the perovskite film is orientated in a direction corresponding to the (001)-plane of the YSZ film can be seen. However, the spontaneous polarization direction of a tetragonal perovskite film is the  less than 001 greater than -direction. If the (011)-plane of a perovskite film is orientated, the spontaneous polarization direction is tilted by 45 degrees with respect to the substrate surface. In such a case, the apparent polarization in the direction perpendicular to the substrate surface decreases, which is disadvantageous in application to a FeRAM or a piezoelectric actuator.
Conventionally, it has been known that an oxide thin film containing a rare earth element such as cerium oxide (CeO2) or yttrium oxide (Y2O3) can be formed through epitaxial growth on a single-crystal silicon substrate by a pulse laser deposition technique using a composite material of the rare earth element as a target. However, such an oxide thin film containing a rare earth element is (011)-orientated with respect to a single-crystal silicon substrate. For this reason, it is difficult to form a (001)-orientated perovskite film through epitaxial growth on such an oxide thin film.
There is also a known method of forming a MgAl2O4 film through epitaxial growth on a single-crystal silicon substrate by a CVD technique. As disclosed in J.Appl.Phys. vol. 66 (1989) pp. 5826, a perovskite film formed on a MgAl2O4 film in this manner has the  less than 001 greater than -direction aligned with respect to the  less than 001 greater than -direction of MgAl2O4 film. This is advantageous in application to a FeRAM or a piezoelectric actuator.
As described above, the only known perovskite oxide film that can be formed through epitaxial growth directly on a single-crystal silicon substrate is a strontium titanate film formed with a thin intermediate layer. Also, the methods of producing such a film only includes a MBE technique that requires a high vacuum of 10xe2x88x9212 Torr or lower, and a pulse laser deposition technique that also requires a high vacuum. A high-vacuum pulse laser deposition technique requires a vacuum of 10xe2x88x928 Torr or lower, and, therefore, a metal-sealed vacuum device is necessary to perform high-vacuum pulse laser deposition. Furthermore, such a high-vacuum process requires maintenance such as baking, and lowers the throughput, resulting in an increase of production costs.
In the MBE technique, metal strontium is used as a raw material. However, an alkaline earth metal such as metal strontium quickly reacts to water, and therefore needs to be stored in oil. With metallic magnesium, there is also a problem in safety, because metallic magnesium easily starts fire. Meanwhile, strontium oxide is used as a target used in a high-vacuum pulse layer deposition technique. However, an oxide containing an alkaline earth metal such as strontium oxide has deliquescence, and absorbs moisture from the air to change into a hydroxide. Such a hydroxide containing an alkaline earth metal is a strong alkali and therefore corrodes the device. As described so far, with the conventional methods and techniques, there are problems in safety and maintenance, as intensive care must be taken in handling raw materials.
In the MBE technique, it is difficult to form a thick intermediate layer such as a perovskite film or a strontium oxide film, because each layer is atomically laminated on one another. In the CVD technique, on the other hand, an organic metal material is supplied in the existence of oxygen, and is decomposed on the substrate surface so as to form a deposition film on the silicon substrate. A cyclepentadiene compound is often employed as the organic metal material. Since such a material does not contain oxygen atoms, however, it is difficult to combine with oxygen at the time of decomposition, often resulting in precipitation of the metal. While an oxide does not easily react to metal, two metallic materials easily diffuse into the substrate to form an alloy. Such a reaction layer disturbs the surface crystal, and therefore hinders the epitaxial growth of a single-crystal oxide thin film on the surface. Further, it is difficult to maintain a high-volume chamber at a high vacuum in accordance with the MBE technique and the high-vacuum pulse laser deposition technique. As a result, it is also difficult to form an oxide film through epitaxial growth on a single-crystal silicon substrate having a large diameter.
Japanese Laid-Open Patent Application No. 6-122597 discloses a structure in which an organic salt such as carbonate, nitrate, or sulfate is used as a target for pulse-laser deposition of an oxide that is unstable in the air at room temperature. In accordance with this technique, an inorganic salt contained in the target is decomposed by laser irradiation, and adheres onto the substrate as a crystalline oxide thin film. In this technique, however, the non-decomposed part of the inorganic salt may also adhere onto the substrate, or a nitride oxide gas or sulfur oxide gas may react to the silicon substrate. To obtain a high-quality crystalline thin film, it is preferable to perform film formation in a vacuum space. However, an inorganic salt often contains water of crystallization in the crystals. When decomposed by laser irradiation, such crystallization water greatly reduces the vacuum. The above facts disturb the crystal structure of the oxide, and become a hindrance to formation of a high-quality crystalline thin film.
A general object of the present invention is to provide semiconductor devices, methods of forming an epitaxial film, and laser ablation devices in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a method of forming a single-crystal oxide thin film through epitaxial growth on a single-crystal silicon substrate, and a semiconductor device that includes such a single-crystal oxide thin film formed on a single-crystal silicon substrate.
Another specific object of the present invention is to provide a method of forming a single-crystal oxide epitaxial thin film having a perovskite structure with a high crystal orientation on a single-crystal silicon substrate at such a vacuum that can be attained by an O-ring seal generally used in a simple vacuum device.
The above objects of the present invention are achieved by a semiconductor device that includes: a single-crystal silicon substrate; and a single-crystal oxide thin film having a perovskite structure formed through epitaxial growth on the single-crystal silicon substrate. In this semiconductor device, the single-crystal oxide thin film is directly in contact with a surface of the single-crystal silicon substrate, and contains a bivalent metal that is reactive to silicon.
The above objects of the present invention are also achieved by a semiconductor device that includes: a single-crystal silicon substrate; a single-crystal oxide thin film having a perovskite structure formed through epitaxial growth on the single-crystal silicon substrate; and an amorphous silicon layer interposed between the single-crystal silicon substrate and the single-crystal oxide thin film.
The above objects of the present invention are also achieved by a semiconductor device that includes: a single-crystal silicon substrate; a first single-crystal oxide thin film having a sodium chloride structure formed through epitaxial growth on the single-crystal silicon substrate; and a second single-crystal oxide thin film having a perovskite structure formed through epitaxial growth on the first single-crystal oxide thin film. This semiconductor device is characterized by the first single-crystal oxide thin film selected from the group consisting of CaO, SrO, and BaO.
The above objects of the present invention are also achieved by a semiconductor device that includes: a single-crystal silicon substrate; a first single-crystal oxide thin film having a sodium chloride structure formed through epitaxial growth on the single-crystal silicon substrate; a second single-crystal oxide thin film having a perovskite structure formed through epitaxial growth on the first single-crystal oxide thin film; and an amorphous layer interposed between the single-crystal silicon substrate and the first single-crystal oxide thin film.
The above objects of the present invention are also achieved by a method of forming an epitaxial film, which method includes the steps of: forming a plume by irradiating a target containing a bivalent metal carbonate with a laser beam; developing a bivalent metal oxide film from the bivalent metal carbonate through epitaxial growth on a single-crystal silicon substrate set in a passage of the plume; and heating a surface of the target with an independent heat source different from the laser beam, thereby producing a single-crystal oxide epitaxial film.
The above objects of the present invention are also achieved by a laser ablation device that includes: a processing chamber that is exhausted by an exhausting system; a processed substrate that is held within the processing chamber; a target that is provided in the processing chamber and faces the processed substrate; an optical window that is provided in the processing chamber and corresponds to an optical path of the laser beam irradiating the target; and a heat source that is provided in the processing chamber and covers a space between the processed substrate and the target.
In accordance with the present invention, when a perovskite oxide film is formed through epitaxial growth on a single-crystal silicon substrate by a laser ablation technique, with a single-crystal oxide film having a sodium chloride structure formed as an intermediate layer, carbonate is used as an ablation target for the oxide film as the intermediate layer. The surface of the target is heated with a heat source other than a laser beam, or, more preferably, the plume itself is heated by the heat source, so that an oxide film containing a metal reactive to the silicon substrate can be formed as the intermediate layer through epitaxial growth. A perovskite oxide film is then formed through epitaxial growth on such an intermediate layer, thereby forming a perovskite single-crystal oxide thin film containing a bivalent metal reactive to silicon on the silicon substrate.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.